The present invention is directed to a novel polyurethane elastomer composition having high wear resistance and low compression set characteristics. It has particular application in an electrostatographic printing apparatus and in particular as a cleaning blade in cleaning an electrostatographic imaging surface.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and printing application from electronically generated or stored originals, and in ionography.
In a reproduction process of the type as described above, it is inevitable that some residual toner will remain on the photoconductive surface after the toner image has been transferred to the sheet of support material (e.g. paper). It has been found with such a process that the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces and, therefore, some of the particles remain on the surface after transfer of the toner image. In addition to the residual toner, other particles, such as paper debris (i.e. kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer. The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid its interfering with recording a new latent image thereon.
Various methods and apparatus may be used for removing residual particles from the photoconductive imaging surface. Heretofore, cleaning brushes, and cleaning webs have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface. After prolonged usage, however, both of these types of cleaning devices become contaminated with toner and must be replaced. This requires discarding the dirty cleaning devices. In high-speed machines this practice has proven not only to be wasteful but also expensive.
Another more widely used device for cyclically cleaning residual toner and other debris from an electrostatographic imaging surface is a cleaning blade which may be used in either the chisel or wiper orientation relative to the imaging surface. Typically, blade cleaning systems are very much less expensive than brush cleaning systems. Such cleaning blades are typically made from non-abrasive flexible elastomeric materials such as polyurethane rubbers. While capable of performing adequately they suffer certain deficiencies, particularly with respect to the variation in their mechanical properties with changes in temperature and relative humidity. The mechanical properties such as resiliency, compression set and tensile set are dynamic properties varying with changes in temperature and relative humidity, thereby, providing unstable cleaning performance. For example, swings in temperature can have a significant dynamic effect on cleaning blade edge performance which is magnified by the frictional heating of the portion of the blade edge riding on the imaging surface or the mere presence or absence of toner on the imaging surface which provide different levels of friction and thereby heating. Furthermore, typical conventional polyester polyurethane blades tend to take a compression set of up to 25 percent after only 24 to 48 hours of frictional contact with the imaging surface which alters the functional set of design parameters of the blade as it relates to the imaging surface such as the attack angle and force of the blade edge relative to the imaging surface. In addition, conventional polyurethanes tend to suffer a wearing deficiency by being susceptible to gouging on the blade edge leading to non-uniform cleaning of the imaging surface and resulting in streaks on the imaging surface and ultimately streaks in the final copy or print. Accordingly, there is a need for an improved material having a more stable response in mechanical properties, particularly resiliency, to variations in temperature and relative humidity and in particular, having better wear and compression set characteristics in cleaning blade applications in electrostatographic apparatus.